Composite material comprising metallic alloy grains coated with a dielectric substance

ABSTRACT

A composite material is provided which includes a discrete phase including grains made of a first substance; and a continuous phase including a thin coating film made of a second substance and formed on the surface of each of the grains. The thin coating film has a mean thickness smaller than the mean particle size of the grains. The grains are separated substantially from each other by the thin coating film. The porosity of the composite material is 5% or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.07/535,080, filed Jun. 8, 1990, now U.S. Pat. No. 5,183,631.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inorganic composite material whichis suitable for many applications such as electronic and structuralmaterials, and to a method for producing the composite material.

2. Description of the Prior Art

Various materials with properties appropriate for their particularapplications are required in the manufacture of electronic components,structural parts and other products, and composite materials combiningthe characteristics of two or more substances are often used in order toobtained the required properties. For example, those such asresin-ceramic and metal-ceramic composite materials are widely used.However, the aforesaid resin-ceramic composite materials have thedisadvantage of low rigidity. On the other hand, metal-ceramic compositematerials have, for example, the form of metal grains 101 dispersed in aceramic matrix 102, as shown in FIG. 7. The proportion of ceramicconstituents in such materials is large, and the metal grains onlyoccupy at most about 50% by volume. The ceramic properties and metallicproperties of such composite materials are only manifested to an extentcorresponding to the proportions of ceramic material and metal,respectively, which are contained in the mixture. Furthermore, althoughthe rigidity of such composite materials is relatively high, thedistances between the grains dispersed in the ceramic matrix is large,and consequently the mechanical strength of such composites is extremelylow. Moreover, triangular regions 103 (i.e., regions in thecross-section bounded by at least three grains; triple point of grainboundaries) occupy a relatively large area in articles formed from suchcomposite materials, and numerous pores are present in these triangularregions. Corrosion may begin from these triangular regions, andconsequently cracks may occur. Hence, such composite materials tend toexhibit poor weather resistance and low mechanical strength.

In addition to the above-described dispersed type of composite material,for example, multilayer laminated structures composed of thin metalfilms and thin ceramic films are well known as composite materialsapplicable to the fabrication of magnetic cores. However, if therelative density of 95% is provided to raise the mechanical strength ofsuch a composite material, then plastic deformation will destroy thelaminated structure, and consequently, as in the case of theabove-mentioned dispersed type of composite material, both the ceramicproperties and metallic properties are only manifested to an extentcorresponding with the proportions of ceramics and metal, respectively,contained in the material. For example, no material developed so far hasthe sufficient magnetic properties of magnetic metals, while at the sametime adequately exhibiting the electrical insulation propertiesassociated with ceramics.

SUMMARY OF THE INVENTION

The composite material of this invention, which overcomes theabove-discussed and numerous other disadvantages and deficiencies of theprior art, comprises a discrete phase including grains made of a firstsubstance; and a continuous phase including a thin coating film made ofa second substance and formed on the surface of the grains, the thincoating film having a mean thickness smaller than the mean particle sizeof the grains, wherein the grains are separated substantially from eachother by the thin coating film and the porosity of the compositematerial is 5% or less.

In a preferred embodiment, the above-mentioned first substance is ametal, more preferably a magnetic metal, and the above-mentioned secondsubstance is a dielectric.

In a more preferred embodiment, the first substance contains an aluminummetal and the second substance contains an aluminum oxide or aluminumnitride.

In a more preferred embodiment, the above-mentioned second substancecontains at least one selected from the group consisting of boron, lead,vanadium, and bismuth compounds.

In a preferred embodiment, the mean thickness of the above-mentionedthin coating film is from 5 to 50 nanometers.

In a preferred embodiment, the above-mentioned first substance is ametal and the above-mentioned second substance is an insulating orhighly electric-resistive material comprising super-plastic ceramics.

In a more preferred embodiment, the super-plastic ceramics contain atleast one of the group consisting of apatite and zirconium oxide.

In a preferred embodiment, the above-mentioned first substance is amagnetic metal such as an iron-based magnetic metal and theabove-mentioned second substance is a highly electric-resistive softmagnetic material such as manganese-zinc ferrite or nickel-zinc ferrite.

In a preferred embodiment, the above-mentioned first substance is aniron-based magnetic metal containing aluminum and the above-mentionedsecond substance is at least one selected from the group consisting ofaluminum oxide, aluminum nitride, and aluminum oxide nitride.

In a preferred embodiment, the above-mentioned grains have a plateletshape.

The method for producing a composite material of this invention, whichovercomes the above-discussed and numerous other disadvantages anddeficiencies of the prior art, comprises the steps of providing grainsmade of a first substance; forming a thin coating film on the surface ofthe grains to prepare coated grains, the thin coating film being made ofa second substance and having a mean thickness smaller than the meanparticle size of the grains; compacting the coated grains into a greenbody; and densifying the green body, while forming an additional thincoating film on the uncoated surface of the grains, the additional thincoating film being made of the second substance or a third substance andhaving a mean thickness smaller than the mean particle size of thegrains.

In a preferred embodiment, the above-mentioned thin coating film isformed by heat treatment in an active gas atmosphere.

In a more preferred embodiment, the above-mentioned treatment isnitridation in an atmosphere which contains nitrogen.

In a more preferred embodiment, the above-mentioned treatment isoxidation in an atmosphere which contains oxygen.

In a preferred embodiment, the above-mentioned thin coating film isformed by heat treatment in air.

In a preferred embodiment, the above-mentioned thin coating film isformed by means of a sputtering treatment.

In a preferred embodiment, the above-mentioned thin coating film isformed by a mechanical alloying treatment.

In a preferred embodiment, the above-mentioned green body is densifiedat a temperature of 300° C. or higher under a pressure of 100 kg/cm² ormore.

In a more preferred embodiment, the pressure is applied to the greenbody in one direction to deform the grains into a platelet shape.

In a preferred embodiment, the coated grains mentioned above are mixedwith a sintering aid and compacted into a green body which is thendensified by a sintering treatment.

In a more preferred embodiment, the sintering aid is at least oneselected from the group consisting of boron, lead, vanadium, and bismuthcompounds.

In a preferred embodiment, the above-mentioned grains are heat-treatedin an atmosphere containing an active gas to form a thin coating film onthe surface of the grains until an increase in the weight of the coatedgrains falls in the range of 0.01 to 2.5%, and the coated grains areencapsulated into an airtight vessel containing an active gas and thenheat-treated under hydrostatic pressure to form a composite material.

Thus, the invention described herein makes possible the objectives of(1) providing an inorganic composite material, composed of two or moresubstances, which can exhibit the desired properties of the constituentsubstances in a synergistic rather than merely an additive manner; (2)providing a composite material which is of high density, rigid, and hasexcellent mechanical strength, thermal resistance, and weatherresistance; (3) providing a composite material having certain desirablecombinations of particular characteristics, for example, compositematerial which consists of 99.9% metal but is also an electricalinsulator, or composite material which consists of 99.9% glass but alsocuts off infra-red rays and has excellent fracture resistance; and (4)providing a method for producing a composite material having thedesirable characteristics mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects andadvantages will become apparent to those skilled in the art by referenceto the accompanying drawings as follows:

FIG. 1 is an enlarged partial sectional view showing the structure of acomposite material of this invention.

FIG. 2 is an enlarged partial sectional view showing the structure ofanother composite material of this invention.

FIG. 3 is an enlarged partial sectional view showing the structure ofstill another composite material of this invention.

FIG. 4 is a graph showing the relationship between the magneticpermeability and the film thickness of a composite material of thisinvention.

FIG. 5 is a graph showing the relationship between the electricalresistivity of the composite material and the volume percent occupied bythe thin coating films in the composite material.

FIG. 6 is a graph showing the relationships between the magneticpermeability and the frequency with respect to the composite materialsobtained in Examples 11 to 13 of this invention.

FIG. 7 is an enlarged partial sectional view showing a conventioncomposite material.

DETAILED DESCRIPTION OF THE INVENTION

The composite material of the present invention comprises a discretephase including grains made of a first substance, and a continuous phaseincluding a thin coating film made of a second substance. The thincoating film is formed on the surface of each of the grains and has amean thickness smaller than the mean particle size of the grains. Thegrains constituting the discrete phase are separated substantially fromeach other by the thin coating film.

The aforesaid first and second substances are appropriately selected inaccordance with the properties required in the final composite material.For example, the first and second substances can be selected from thegroup consisting of inorganic substances such as metals, metal oxides,metal nitrides, and ceramics. For example, as the first and secondsubstances, a metal (i.e., aluminum) and a dielectric (i.e., aluminumoxide or aluminum nitride) may be selected, respectively, therebyobtaining a composite material combining the properties of these twosubstances. Excellent magnetic materials can be obtained by selecting amagnetic metal as the first substance. For example, as the firstsubstance, materials with high magnetic permeability such as Si-Al-Fe,Fe-Al, Fe-Ni, or Mo-Ni-Fe alloys, may be used which can provide highsaturated magnetic flux density. Soft magnetic materials with electricalinsulating properties or high electrical resistivity, such as Mn-Znferrite or Ni-Zn ferrite, are suitable for use as he second substance.Compounds such as aluminum oxide, aluminum nitride, or aluminum oxidenitride are also applicable for this purpose. By appropriately selectingthe aforesaid types of materials, there can be obtained compositematerials capable of attaining high saturated magnetic flux density andalso having electrically insulating surfaces.

The production of the composite materials comprises preparing coatedgrains by forming a thin coating film of the second substance on thesurfaces of grains made of the first substance; compacting these coatedgrains into a green body; and densifying the green body. Any of thewell-known methods can be used to form the thin coating film made of thesecond substance. For example, there may be employed a method ofallowing grains made of the first substance to come into contact with anactive gas, whereby the active gas reacts with the first substance onthe surfaces of the grains, thus forming a layer of a substancedifferent from the original first substance on the grain surfaces; or amethod using a sputtering procedure to form a layer of the secondsubstance on the surfaces of grains made of the first substance; or amethod using a mechanical alloying process to deposit a layer of thesecond substance on the grain surfaces. The layer of the secondsubstance should have a mean thickness smaller than the mean particlesize of the grains made of the first substance; the appropriate sizes ofthe grains and thin coating film will vary according to the particularpurpose and type of composite material, but ordinarily the range of meanparticle sizes is 0.1-100 μm while the mean thickness of the thincoating film is 5-50 nm.

Specific examples of the formation of the thin coating film, include theformation of metal oxide films on the surfaces of metal grains byoxidation treatment, as well as the formation of thin coating films ofsome different metal on the surfaces of metal grains by sputtering, etc.

Next, the coated grains prepared by using any of the above methods arecompacted into a green body which is then subjected to a densificationprocess, thereby obtaining the composite material of the presentinvention. For example, if the grains are made mainly of ceramics (e.g.,if the grains of the first substance are of ceramic composition), thenthe material can be compacted into the desired shape and then densifiedmerely by sintering. In general, densification can be achieved byhigh-temperature and/or high-pressure treatment. Ordinarily,densification is accomplished by treatment at a temperature of 300° C.or higher under a pressure of 100 kg/cm² or higher. When sintering isperformed, sintering aids may be added, if necessary. The sintering aidswhich can be used include compounds of boron, lead, vanadium, orbismuth, such as boron oxide (B₂ O₃), lead oxide (PbO), vanadiumpentoxide (V₂ O₅), and bismuth trioxide (Bi₂ O₃). For example, ifaluminum powder with aluminum oxide films on the surfaces of the powdergrains is prepared and one of these sintering aids is added prior tosintering, then the melting point of the aluminum oxide is lowered, andtherefore the aluminum oxide film is more easily softened during thesintering process. Consequently, the sintering is enhanced and thedensity of the sintered body so obtained (i.e., the desired compositematerial) is increased. Hence, the mechanical strength and toughness ofthe sintered body is improved. Similarly, if the first substance is ametal, then the workability and thermal conductivity of the compositematerial are improved. The reaction products formed from the sinteringaids during the sintering treatment remain intermixed with the thincoating film made of the second substance in the composite materialobtained.

In the step of densification, a portion of the thin coating films on thegrains may in some cases be broken by the operations of compression,etc., thereby exposing the grain surfaces made of the first substance.Therefore, densification is ordinarily performed in such a manner that athin coating film is again formed by the second substance or some thirdsubstance. For example, if the densification is performed in anatmosphere of an active gas, the first substance on the exposed surfacesreacts with the active gas, thereby forming an additional thin coatingfilm. As the active gas, oxygen or nitrogen, for example, may be used. Amixture of these active gases may also be used.

By selecting a metal as the first substance and an electrical insulatoras the second substance, a composite material combining metallicproperties and highly effective electrical insulation characteristicscan be obtained. In such cases, the following method can also beemployed. Firstly, a covering phase made of the second substance isformed on the surfaces of grains made of the first substance, and usingthese coated grains, green bodies with a relative density of 80-95% areprepared. The sintered bodies are prepared so as to have open pores.Next, these sintered bodies are brought into contact with anelectrically insulating substance, or a substance which can generate anelectrical insulator, or a substance which can react with the metal andthereby form some electrically insulating substance, and are then onceagain densified under high pressure to form a composite material with arelative density of 95% or greater. For example, Fe-Si-Al, Fe-Ni, Fe-Si,Fe-Al, or Mo-Ni-Fe are used as the first substance, and oxide films areformed on the surfaces of grains made of this first substance. Next,these grains are brought into contact with a substance, such as oxygenor titanium ethoxide, which can react with the first substance, thereby,forming an electrically insulating substance, while simultaneouslyperforming compression molding, and thus obtaining a composite materialwhich has metallic properties as well as effective electrical insulationcharacteristics. Moreover, the following method of manufacturingmagnetic core materials may be mentioned as an application of theaforesaid method. First, grains made of a magnetic alloy are subjectedto oxidizing treatment to form an oxide coating on the grain surfaces.This is done in such a manner that the increase in weight amounts to0.01-2.5%. These coated grains are then encapsulated into an airtightvessel containing an oxygen gas, and heat-treated under hydrostaticpressure until the porosity is reduced to 3% or less. By this procedure,magnetic core materials having magnetic properties of the same order asthose of conventional laminated magnetic core materials can be obtained.

If a metal is used as the first substance, and if an electricallyinsulating or highly electric-resistive material containing inorganicceramics exhibiting super-plasticity is used as the second material,then the aforesaid super-plastic inorganic ceramics conform well withthe deformations of the metal grains during the molding process.Consequently, high densification is attained, and moreover, breakdown ofthe continuous phase composed of the second substance (in this case, anelectrically insulating layer) does not occur. Super-plastic inorganicceramics applicable for the present purpose include apatite, zirconiumdioxide (ZrO₂), bismuth trioxide (Bi₂ O₃), magnesium oxide (MgO), anduranium dioxide (UO₂). The apatite is a mineral of the generic formulaX₅ (YO₄)₃ Z, where X is Ca²⁺, Mg²⁺ or Pb²⁺, and Y is P⁵⁺ or As⁵⁺, and Zis F⁻, Cl⁻, OH⁻. In particular, if a material containing at least one ofthe apatite and zirconium dioxide is used, then, since these substancesexhibit super-plasticity even at relatively low temperatures, theaforesaid advantage can be fully attained.

The composite materials formed in this manner have, for example, thestructure shown in FIG. 1, where the grains 1 made of the firstsubstance are dispersed as a discrete phase in a continuous phase 2consisting of a thin coating film formed from the second substance.Likewise, composite materials with the structure shown in FIG. 2 canalso be obtained by compacting and sintering grains made of a substanceeither identical with or entirely different from the first or secondsubstance together with the aforesaid coated grains. For example, acomposite material of the type shown in FIG. 2 can be formed byselecting aluminum oxide granules as the grains 11 made of the firstsubstance, and forming the continuous phase 12 from aluminum as thesecond substance. In FIG. 2, the face 10 composed of the first substanceexhibits complete electrical insulation characteristics, while theopposite face 100 where the first and second substances are intermingledwith each other is electrically conductive.

A composite material with the structure shown in FIG. 3, containing adiscrete phase consisting of platelet grains 21 made of the firstsubstance and a continuous phase 22 consisting of thin coating filmsmade of the second substance, can be obtained by applying pressure inonly one direction during the densification process employed in thepreparation of the composite material. Also, platelet grains can be usedfrom the start for the preparation of the composite material. Thecomposite materials obtained exhibit high hardness along the directionof the longitudinal axes of the grains, and superior wear resistance. Inall of the aforesaid composite materials, the cross-sectional areaoccupied by triangular regions (i.e., regions bounded by three or moreparticles, indicated in the FIGS. 1-3 by reference numerals 3, 13, and23, respectively) is small as compared with conventional types ofcomposite materials, and therefore the composite materials of thepresent invention exhibit superior mechanical strength. The porosity ofthe composite materials of the present invention should ordinarily be 5%or less (i.e., the density thereof being 95% or more), and preferably 3%or less. If the porosity is 5% or less, then the existing pores will beclosed and isolated from each other in the composite material, and theinterior of the composite material will not communicate with theexterior. Therefore, the composite material will exhibit high mechanicalstrength as well as superior weather and chemical resistances. Thecomposite materials obtained in this manner have a synergisticcombination of the properties of the first and second constituentsubstances. For example, if the first substance is a metal and thesecond substance is a metal oxide, then the composite material will haveboth metallic properties and dielectric characteristics. For example,using the method of the present invention, it is possible to producecomposite materials composed of 99.9% metal, while also havingelectrically insulating characteristics.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be further explained by reference to the followingexamples.

EXAMPLE 1

A powder consisting of grains made of Fe-5% Al alloy (first substance),with a mean particle size of 30 μm, was oxidized by heating in air at800° C. for one hour, thereby forming a uniform thin coating film ofaluminum oxide (second substance) on the surfaces of the alloy grains.Aluminum oxide is a dielectric, and accordingly the coated grainsexhibited complete electrically insulating characteristics. This powderconsisting of coated grains was then mixed with 0.05 wt % of a waxvolatile at low temperatures as a binder, molded, and, after removal ofthe binder, sintered in air at a temperature of 1350° C. under apressure of 50 kg/cm² for one hour. The sintered body (compositematerial) so obtained was sufficiently dense, with a porosity of about4.2%.

When this sample was cut and polished, the surface thereof is a mirrorsurface, displaying a metallic luster which manifested thecharacteristics of the Fe-Al alloy constituting the first substance inthe composite material. The areal proportion of triangular regions wasfound to be approximately 3%. Owing to the compression of the grains,the mean size of these triangular regions was extremely small (i.e.,approximately 1.2-2 μm), or less than about 1/15 of the mean particlesize of the grains made of the first substance. The electricalresistance of this polished surface was measured by applying a tester,and found to be approximately 15 megohm. Thus, whereas the electricalresistance of a sintered body prepared from a Fe-Al alloy containing 5%aluminum without forming an oxide film exhibited an electricalresistance nearly equal to zero ohm, this composite material, preparedby the method of the present invention, exhibited a metallic lustercombined with extremely high electrical resistance, propertiesordinarily regarded as incompatible in conventional materials. Thehardness of the composite material obtained in this way was also greatlyimproved, being approximately 30% higher than that of samples formedfrom the Fe-5% Al alloy in the conventional manner. Moreover, thetemperature limit indicating the thermal resistance of the compositematerial was not merely of the order of about 100° C., as in the case ofconventional resin-based composite materials, and in fact no problems ofthermal resistance arose even at a temperature of 1000° C.

The composite material obtained in the aforesaid manner was loaded intoa thermal-resistance vessel, and the vessel was charged with an activegas (oxygen or nitrogen) at ordinary pressure. This sample was thenheated at 800° C. for one hour, and then subjected to hot pressing undera pressure of 2000 kg/cm² for two hours, thereby obtaining ahigh-density sintered body with a porosity of 0.1% or less. Analysisrevealed that the continuous phase of the final sintered body had beenmade entirely of oxides or nitrides. The areal proportion of triangularregions in the cross-section was 0.1% or less. The electrical resistanceof this sample at the cross-section thereof was 20 megohm or more, abovethe limit of measurement of the tester, while the hardness wasapproximately 25% higher than that prior to the hot pressing.

In another experiment, the composite material obtained in the aforesaidmanner was loaded into a thermal-resistance vessel, and the vessel wascharged with an inactive gas (e.g., argon) at ordinary pressure. Thissample was then heated at 800° C. for one hour, and then subjected tohot pressing under a pressure of 2000 arm for two hours, therebyobtaining a high-density sintered body with a porosity of 1.5% or less.However, the electrical resistance of this sample at the cross-sectionthereof was 1 ohm or less, suggesting that the films at the grainboundaries had been broken. This sort of destruction was also observedby electron microscopy.

EXAMPLE 2

First, a pure iron powder consisting of grains with a mean particle sizeof 15 μm was sputtered with aluminum to form a thin coating film ofthickness approximately 0.05 μm (i.e., 50 nm) on the grain surfaces.Using these coated grains, a composite material with a porosity of 0.1%or less was obtained by the same process using an active gas asdescribed in Example 1. As in Example 1, this composite material alsoexhibited a high electrical resistance and the area of the triangularregions was also extremely small.

EXAMPLE 3

First, a 0.1% polyvinyl alcohol solution was added to a Fe-Al-Si alloypowder consisting of grains with a mean particle size of 3 μm, and thismixture was molded under a high pressure of 5000 kg/cm². This moldedbody (i.e., green body) was then heated in air at 800° C. for one hour,thereby forming a thin oxide film, composed mainly of Al₂ O₃ (the secondsubstance) on the surfaces of the powder grains. Next, this molded bodywas sintered for three hours in air at a temperature of 800° C. under apressure of 200 kg/cm² in a hot pressing apparatus. From the results ofanother model experiment, the weight increase in the present case wasfound to be 0.1%, showing that 99.9% of the sintered body was unoxidizedmetal. That is, the measurements indicated that the continuous phasecomposed of the second substance was also formed to some extent duringthe sintering process. In this case, the element aluminum constitutingthe first substance was oxidized by the active gas, oxygen, thus forminga film of aluminum oxide (the second substance), which is a dielectricwith high electrical insulation characteristics. The porosity of theinorganic composite material obtained in this manner was 5% or less. Thepolished surfaces of samples of this composite material weremirror-like, with a metallic luster, and exhibited electrical resistancevalues above 20 megohm. Moreover, the Vickers hardness of this samplewas extremely high, i.e., above 700 (c.f., sintered bodies prepared inthe same way without forming an oxide film had a Vickers hardness of500-550). Furthermore, the total proportion of triangular regions in thecross-sections of these samples was exceedingly low, i.e., 2.5% or less,and the aluminum oxide films were extremely thin, with a mean thicknessof only about 1/20 of the mean particle size.

Another composite material was also prepared in the same manner, but inthis case with the hot pressing apparatus set to a pressure of 400kg/cm² ; the porosity of this material was 3% or less, the electricalresistance was above 20 megohm, and the Vickers hardness was 720. Whenthe composite material was prepared under a pressure of 800 kg/cm², thehardness was even higher, i.e., 750 or higher.

EXAMPLE 4

First, aluminum powder consisting of grains with a mean particle size of15 μm was subjected to oxidation treatment for two hours at 300° C. inan atmosphere of 5% oxygen. After molding, this material was sintered byhot pressing at 500° C. under a pressure of 500 kg/cm² in an atmosphereof 0.5% oxygen. The inorganic composite material so obtained had ahardness approximately twice that of aluminum, and an electricalresistance equal to or greater than 20 megohm. On the other hand, acomparative sample was also prepared by applying the aforesaid oxidationtreatment to the same powder, followed by hot pressing in a nitrogenatmosphere, and the electrical resistance of this comparative sample wasapproximately 15 megohm. This result showed that oxidation during thesintering process formed films more effectively than sintering undernitrogen gas. Nevertheless, the effectiveness of nitridation was alsodemonstrated by the aforesaid example. Since the hardness of thecomposite materials obtained in this manner is high, these compositematerials require greater force for machining than the originalmaterials, but nevertheless can be machined by the same methods as thoseof conventional aluminum materials.

By applying the aforesaid hot pressing treatment in only one direction(i.e., uniaxial compression), the originally almost spherical grainsmade of aluminum were deformed into platelet grains having a diameter tothickness ratio of at least 2:1. By suitably adjusting the pressure andthe direction of the uniaxial force, the degree of flatness can befurther changed, thereby obtaining platelet grains with a diameter tothickness ratio varying up to about 10:1. Composite materials preparedin this manner exhibit anisotropic hardness and wear resistance, andthese materials are suitable for a wide variety of applications.

EXAMPLE 5

Titanium monohydride powder was added to aluminum powder consisting ofgrains with a mean particle size of 15 μm, and the mixture was blendedin a ball mill for 80 hours to effect surface modification of thealuminum grains. The coated grains so obtained were placed in anatmosphere of nitrogen gas at 550° C., thus converting the surface layerto nitrides. This material was then molded and hot-press sintered at400° C. and 500 kg/cm², thereby obtaining an inorganic compositematerial with a hardness approximately 1.5 Times That of conventionalaluminum materials and an electrical resistance of 20 megohm. Thethermal conductivity of this composite material was high, nearly thesame as that of the conventional aluminum materials.

EXAMPLE 6

Samples of an aluminum metal powder consisting of grains with a meanparticle size of about 20 μm were heat-treated at the varioustemperatures shown in Table 1, thereby forming oxide films of variousthicknesses on the surfaces of the powder grains.

The respective film thicknesses and compositions were determined fromThe weight increase during heat treatment, Auger electron spectroscopy,and argon sputtering depth profiles.

These powder samples were molded, vacuum packed into metal aluminumpipe, and subjected to isotropic compression by argon gas for two hoursat 600° C. and 1000 kg/cm², thereby preparing a high-density compositematerial.

Next, 2×1×12 mm rectangular parallelepipedic samples were cut from thehigh-density composite material obtained in this manner, and thedensity, electrical resistance (measured by a tester), and thermalconductivity of these samples were measured, with the results shown inTable 1.

For comparison, these parameters were also measured by the same methodwith respect to samples of metal aluminum and aluminum oxide sintered inthe ordinary manner.

                  TABLE 1                                                         ______________________________________                                                                       Elec-                                          Heat Treatment                                                                              Film             trical                                                                              Thermal                                  Temper-               Thick-       Resit-                                                                              Conduc-                              ature  Time   Atmo-   ness  Density                                                                              ance  tivity                               (°C.)                                                                         (hr)   sphere  (nm)  (g/cm.sup.3)                                                                         (ohm) (W/m-deg)                            ______________________________________                                        Not heat treated                                                                             2      2.63     <0.1  190                                      400    1      air      5    2.67   180   190                                                              (98.9%)                                           450    1      air     18    2.62    32 k 180                                                              (97.0%)                                           500    1      air     35    2.59   780 k 160                                                              (95.9%)                                           600    1      air     47    2.57   >20M  150                                                              (95.2%)                                           600    50     air     68    2.53   >20M  110                                                              (93.7%)                                           620    50     air     130   2.45   >20M   80                                                              (90.7%)                                           --     --     --      --    .sup.  2.70.sup.1)                                                                   <0.1  200                                  --     --     --      --    .sup.  3.91.sup.2)                                                                   >20M   20                                  ______________________________________                                         Note:                                                                         .sup.1) Sample of metal aluminum sintered in the ordinary manner was used     .sup.2) Sample of aluminum oxide sintered in the ordinary manner was used                                                                              

The results in Table 1 show that, for film thicknesses less than 5 nm,the density, electrical resistance, and thermal conductivity of theresulting composite materials were nearly equal to those of the metalaluminum sample tested for comparison. When the film thickness was 5 nmor greater, although the density was slightly lower than that ofconventional aluminum materials, the thermal conductivity exhibited nogreat change. However, the electrical resistance rose markedly. When thefilm thickness was 50 nm or greater, the electrical resistance wasextremely high, while the thermal conductivity dropped to the order ofhalf the thermal conductivity of metal aluminum, and the density droppedbelow 2.5 g/cm³.

A regards materials used as insulating substrates for electricalcomponents, high electrical resistance is desirable, but if the thermalconductivity is less than half the thermal conductivity of ordinarymetals, then the material of interest can hardly be regarded asparticularly superior in comparison with conventional substratematerials. Moreover, if the density is low (i.e., below 95%), thennumerous problems relating to practical applications arise, such aslarge numbers of open pores and low mechanical strength. Therefore, evenif the electrical resistance is slightly low, if the thermalconductivity is comparable with that of ordinary metals, then thematerial can be used for purposes such as the slider parts of magneticheads.

The electrical resistivity of insulating metallic oxide films is high,exceeding 10¹² ohm-cm, but the overall electrical resistance values ofthe aforesaid composite materials obtained from coated grains with filmsof the order of 5-20 nm in thickness were smaller than those expectedfrom the aforesaid resistivity values of metal oxide films. Electronmicroscopic examination of samples of these composite materials clearlyrevealed the presence of metallic oxide films at the grain boundaries,hence, the surfaces of the metal grains were almost covered by aninsulator. However, the slightly low resistance values mentioned abovemay possibly be attributed to the existence of pin-holes in parts of theinsulator, allowing partial contact between metal grains over a smallarea.

Accordingly, the coated grains were loaded into an aluminum pipetogether with oxygen gas, and hot pressing was applied in the samemanner as described above. As a result, even for samples with filmthicknesses of 5-50 nm, the electrical resistance values were elevatedby a factor of 1.5-100, while the densities remained unchanged. Thus,additional formation of the second substance (in this case, aluminumoxide) during the densification treatment is also desirable.

EXAMPLE 7

The first substance used in this example was a spherically granularpowder (mesh #250 or less, mean particle size approximately 30 μm)composed of a Si-Al-Fe alloy containing 10 wt % of silicon, 6 wt % ofaluminum and 84 wt % of iron. To this granular powder was added asuper-plastic second substance (bismuth trioxide, magnesium oxide, oruranium dioxide) in a proportion of 1 wt % relative to the weight of theSi-Al-Fe alloy. After mixing, this mixture was placed in a ball mill anduniformly blended with toluene as a solvent. This slurry was mixed anddried in a stream of nitrogen gas. Next, 5 wt % of camphor, as asublimable organic binder, was added and the mixture was further blendedin a mortar.

This mixed powder was then molded by isostatic compression under ahydrostatic pressure of 1000 kg/cm² into a disc of diameter 30 mm andthickness 30 mm. Next, this molded sample was hot pressed at atemperature of 800° C. in an atmosphere of inactive gas, therebypreparing a high-density sintered body; the pressure used was 1000-2000kg/cm² and the pressing time was two hours.

The density and electrical resistance of the sintered bodies obtained inthis manner were measured by the following methods.

A cross-section of each sintered body was polished, and opticalmicroscopic observation revealed that these sintered bodies werecomposed of extremely dense material with a porosity of 1% or less.Also, 3×5×10 mm rectangular parallelepipedic samples were cut from eachsintered body, and indium-gallium electrodes were formed on the two endsof each sample, whereby the electrical resistivity was measured andfound to be 10-100 ohm-cm.

EXAMPLE 8

The first substance used in this example was the spherically granularpowder composed of an Si-Al-Fe alloy (the first substance) which wasemployed in the preparation of Example 7. To this granular powder wasadded a super-plastic electrically insulating material (secondsubstance), i.e., polycrystalline tetragonal zirconia containing 3 mol %of yttrium oxide (referred to below as Y-TZP) in a proportion of 1 wt %relative to the weight of the Si-Al-Fe alloy. This mixture was placed ina ball mill and uniformly blended with toluene as a solvent. This slurrywas mixed and dried in a stream of nitrogen gas. Next, 5 wt % ofcamphor, as a sublimable organic binder, was added and the mixture wasfurther blended in a mortar.

This mixed powder was then molded by isostatic compression under ahydrostatic pressure of 1000 kg/cm² into a disc of diameter 30 mm andthickness 30 mm. Next, this molded sample was hot pressed at atemperature of 800° C. in an atmosphere of inactive gas, therebypreparing a high-density sintered body; the pressure used was 1000-2000kg/cm² and the pressing time was two hours.

The density and electrical resistance of the sintered bodies obtained inthis manner were measured by the same methods as those used formeasuring the corresponding parameters in Example 7.

The results showed that these sintered bodies were composed of extremelydense material with a porosity of 1% or less. The electrical resistivitywas found to be 10¹⁰ ohm-cm.

EXAMPLE 9

As a super-plastic electrical insulator (the second substance), in placeof Y-TZP, apatite (Ca₅ (PO₄)₃ OH) was added in a proportion of 1 wt % tothe spherically granular powdered Si-Al-Fe alloy used in the preparationof Example 7, and high-density sintered bodies were prepared from thismaterial by the same process as that used in Example 7.

The density and electrical resistance of sintered bodies obtained inthis manner were measured by the same methods as those used formeasuring the corresponding parameters in Example 7.

The results showed that these sintered bodies were composed of extremelydense material with a porosity of 1% or less. The electrical resistivitywas found to be 10¹⁰ ohm-cm.

Thus, the electrical resistivities of the composite materials obtainedin Example 7 were 10-100 ohm-cm, whereas those of the compositematerials obtained in Examples 8 and 9 were of the order of 10 ohm-cm.This is attributable to the fact that the Y-TZP or apatite used as thesecond substance in the latter two Examples exhibits super-plasticityfrom the low temperature region, and therefore undergoes adequateplastic deformation in response to the plastic deformation of theSi-Al-Fe alloy powder grains.

Thus, high-density (porosity not more 5%), highly electric-resistancecomposite materials are obtainable if super-plastic substances such asY-TZP or apatite are used as the electrical insulating material.

COMPARATIVE EXAMPLE 1

Using the Si-Al-Fe alloy of Example 7, the super-plastic insulator Y-TZPwas not added, but 5 wt % of camphor was added, molded samples wereprepared by the same procedure as that employed for this purpose in thepreparation of Example 7, and likewise the samples were hot-pressedunder the same conditions.

The density and electrical resistance of the sintered bodies obtained inthis manner were measured by the same methods as those used formeasuring the corresponding parameters in Example 7.

The results showed that these sintered bodies were composed of extremelydense material with a porosity of 1% or less. However, the electricalresistivity was found to be only of the order of 10⁻⁴ ohm-cm, i.e., ofthe same order as that of ordinary metals.

EXAMPLE 10

As a material with high electrical resistance, an aluminum oxide powder(the second substance) was added in the proportion of 1 wt % to theSi-Al-Fe alloy of Example 7, sintered bodies (composite materials) wereprepared by the same procedure as that employed in the preparation ofExample 7, and the sintering density as well as electrical resistance ofthese composite materials was measured.

The results of these measurements showed that all these compositematerials were of high density, with a porosity of 1% or less, however,the electrical resistivities were low, i.e., of the order of 10⁻⁴ohm-cm, that is, of the same order as that of ordinary metals.

In other experiments, silicon dioxide and calcium oxide were selected ashighly electric-resistive materials (electrical insulators) in place ofY-TZP, these oxides were separately added to powdered grains of theaforesaid Si-Al-Fe alloy in the same manner as Y-TZP, sintered bodieswere prepared from these samples by the same procedure as that used inthe preparation of Example 7, and the porosity (sintering density) aswell as electrical resistance of these sintered bodies was measured. Theresults of these measurements showed that the electrical resistivitiesof those composite materials with comparatively low density (porosityabove 5%) were high (>10³ ohm-cm), whereas for those composite materialsof comparatively high density (porosity below 5%), the electricalresistivity decreased with increasing density, owing to greaterdestruction of the insulating layer. Among the comparativelyhigh-density composite materials, samples with electrical resistivitiesranging from the order of 10⁻⁴ ohm-cm (i.e., comparable with ordinarymetals) to the order of 10 ohm-cm were prepared, but this electricalresistivity was subject to considerable variation, and highlyelectric-resistance materials were not obtained with goodreproducibility. Thus, the electrical resistivities of the compositematerials obtained by adding the super-plastic electrical insulators ofExample 7 were at most of the order of 10 ohm-cm, and resistivitieshigher than this could not be obtained with these additives.

Thus, as indicated in the descriptions of Examples 7-9, compositematerials containing a large proportion of metal but neverthelessexhibiting high electrical resistance can be obtained by the method ofthe present invention. These types of composite materials havecharacteristics not obtainable in conventional metal materials. Ingeneral, magnetic metal materials exhibit high saturated magnetic fluxdensity but low electrical resistivity, i.e., values of the order of10⁻⁴ ohm-cm. Consequently, conventional magnetic metal materials couldnot be utilized as magnetic materials in the high-frequency rangebecause of eddy-current losses. However, since, by using about 1 wt % ofa super-plastic electrical insulator, the electrical resistivity can beraised above 10³ ohm-cm without diminishing saturated magnetic fluxdensity, the method of the present invention permits the preparation ofmagnetic core materials which can be used in the high frequency range(e.g., above 1 MHz).

In the preparation of Examples 7-9, super-plastic electrical insulatorswere added to an Si-Al-Fe alloy, however, the procedure describedtherein is by no means limited to Si-Al-Fe alloys, in fact, other metalssuch as Fe-Co alloys, Fe-Si alloys, Fe-Ni alloys, or Fe-Al alloys, etc.,indeed, any metal or intermetallic compound can be used for the presentpurpose. Moreover, the proportion of the added super-plastic substance(second substance) is not restricted to 1 wt %.

Moreover, although camphor was used as a binder for the preparation ofthe molded sample in the processes of Examples 7-9, the binders whichcan be employed for this purpose are not restricted to camphor only. Forexample, any binder which sublimes at high temperatures and pressures,such as polymethyl methacrylate, is also applicable for this purpose.Also, if the sample is loaded into a vessel, then a binder is notnecessarily required.

EXAMPLE 11

A spherically granular powder (mesh #250, mean particle sizeapproximately 30 μm) composed of an Si-Al-Fe alloy containing 10 wt % ofsilicon, 6 wt % of aluminum, and 84 wt % of iron was immersed in anaqueous solution of nickelous chloride, zinc chloride, and ferrouschloride at 70° C., then the mixture was adjusted to pH 7-8 and allowedto react. Thus, metal was precipitated onto the surfaces of powdergrains immersed in this solution, and next this precipitated film wasoxidized in air to effect ferritization.

By repeating the aforesaid two steps, soft magnetic Ni-Zn ferrite filmsof thickness 5-50 nm were formed.

In separate experiments, the magnetic and electrical characteristics ofNi-Zn ferrite films of thickness 5-50 nm, as such, were evaluated usingNi-Zn ferrite films deposited on glass substrates. The results of thesemeasurements showed that these films exhibited saturated magnetic fluxdensities B_(s) of about 3000 gauss, magnetic permeabilities μ (CGS) ofabout 1000, and electrical resistivities ρ ranging from about 10⁵ to 10⁹ohm-cm.

The aforesaid Si-Al-Fe alloy powder coated with soft magnetic ferritewas molded by compression at 500 kg/cm², and then hot pressed innitrogen gas at a temperature of 1000° C. under a pressure of 300 kg/cm²for two hours, thereby obtaining a high-density composite magneticmaterial.

The magnetic and electrical characteristics of this composite magneticmaterial are shown in FIGS. 4 and 5, respectively. FIG. 4 shows therelationship between the thickness of the Ni-Zn ferrite film and themagnetic permeability μ (CGS) at the frequency 100 kHz) of thecorresponding composite material. FIG. 5 shows the relationship betweenthe volume percent occupied by Ni-Zn ferrite films in the compositematerial and the electrical resistivity of the composite material.

From these results, one observes that the magnetic permeability μ (CGS)was 5000-6000, virtually independent of thickness in the range 5-50 nm,while the electrical resistivity ρ ranged from about 10⁵ to 10⁷ ohm-cm.The saturated magnetic flux density B_(s) of this composite magneticmaterial was 9600 gauss, almost identical with that of the originalpowdered material. Similar results were obtained in the case where thesame powder was coated with an Mn-Zn ferrite film.

The magnetic and electrical characteristics of Mn-Zn ferrite films wereevaluated using films deposited on glass substrates. The results ofthese measurements showed that these films displayed saturated magneticflux densities B_(s) of about 5000 gauss, magnetic permeabilities μ(CGS) of about 1000, and electrical resistivities ρ ranging from about 1to 10³ ohm-cm.

EXAMPLE 12

A spherically granular powder composed of the Si-Al-Fe alloy which wasemployed in the preparation of Example 11 was immersed in an aqueoussolution of ethoxysilane and the mixture was thoroughly agitated. Then,this mixture was suction filtered and the residue was dried at 80° C.,thereby forming a nonmagnetic silicon dioxide film of thickness 50 nm onthe surfaces of the powder grains.

This silica-coated Si-Al-Fe alloy powder was molded by compression at500 kg/cm², and the green body so formed was then subjected to hotpressing in an atmosphere of nitrogen gas under a pressure of 300 kg/cm²for a temperature of 1000° C. for two hours, thereby obtaining ahigh-density composite magnetic material.

This composite magnetic material exhibited a high resistivity of 10⁹-10¹⁰ ohm-cm. However, the magnetic permeability μ (CGS) at the thefrequency of 100 Hz was low, i.e., 300. When the silicon dioxide filmwas prepared with a thickness of 5 nm, the electrical resistivity ρ ofthe composite magnetic material so obtained ranged from 10⁶ to 10⁸ohm-cm, and the magnetic permeability μ (CGS) was 2500.

EXAMPLE 13

The spherically granular powder composed of the Si-Al-Fe alloy used invarious examples described above was immersed in an aqueous solutioncontaining nickelous chloride and ferrous chloride at a temperature of70° C., then the mixture was adjusted to pH 7-8 and allowed to react,whereby metal was precipitated from the solution onto the powder grains,and an Fe-Ni (permalloy) film of thickness 50 nm was formed on the grainsurfaces.

This Si-Al-Fe alloy powder, coated with an Fe-Ni film, was then moldedby compression at 500 kg/cm², and the resulting green body was hotpressed in an atmosphere of nitrogen gas at a temperature of 1000° C.under a pressure of 300 kg/cm² for two hours, thereby obtaining ahigh-density composite magnetic material.

In separate experiments, the magnetic and electrical characteristics ofFe-Ni films with a thickness 50 nm, as such, were evaluated using Fe-Nifilms deposited on glass substrates. The results of these measurementsshowed that these films exhibited saturated magnetic flux densitiesB_(s) of about 8700 gauss, magnetic permeabilities μ (CGS) of about20,000, and electrical resistivities ρ of about 60 microhm-cm.

The electrical resistivity of the aforesaid composite magnetic materialwas 100 microhm-cm. Also, the magnetic permeability μ (CGS) of thiscomposite magnetic material was 5500 at the low frequency of 100 Hz and30 at the high frequency of 1 MHz.

These results show that if the electrical resistivity of the compositemagnetic material is low, then the magnetic permeability in the highfrequency range drops abruptly because of eddy-current loss.

EXAMPLE 14

The spherically granular powder composed of the Si-Al-Fe alloy used inthe preparation of Example 11 was immersed in an aqueous solutioncontaining nickelous chloride and ferrous chloride at a temperature of70° C., then the mixture was adjusted to pH 7-8 and allowed to react,whereby metal was precipitated from the solution onto the powder grains.This surface film was then ferritized by oxidation in air.

By repeating the aforesaid step, a NiFe₂ O₄ ferrite film with athickness of 100-1000 nm was formed on the grain surfaces.

In separate experiments, the magnetic and electrical characteristics ofthese NiFe₂ O₄ films with a thickness of 100-1000 nm, as such, wereevaluated using NiFe₂ O₄ films deposited on glass substrates. Theresults of these measurements showed that these films exhibitedsaturated magnetic flux densities B_(s) of about 3400 gauss, magneticpermeabilities μ (CGS) of about 10, and electrical resistivities ρ ofabout 10³ -10⁴ ohm-cm.

This Si-Al-Fe alloy powder, coated with the aforesaid ferrite film, wasthen molded by compression at 500 kg/cm², and the resulting green bodywas hot pressed in an atmosphere of nitrogen gas at a temperature of1000° C. under a pressure 300 kg/cm² for two hours, thereby obtaining ahigh-density composite magnetic material.

The magnetic permeability μ (CGS) of this composite magnetic materialwas 120 at the frequency of 1 MHz, while the electrical resistivity ρwas 10-10² ohm-cm. These measurements show that if the surfaces of thepowder grains are covered with an insulating film of low magneticpermeability and a thickness of 100 nm or more, then the magneticpermeability of the resulting composite material decreases.

As shown by the aforesaid and Examples 11-14, if a material with highelectrical resistivity and soft magnetic properties is selected as thesecond substance referred to above, and if the surfaces of grains of amagnetic metal material are covered with this second substance, then thecomposite materials so obtained have high electrical resistivity as wellas high saturated magnetic flux density and high magnetic permeability.

If a magnetic material with high electrical resistivity is chosen as thesecond substance, then a composite material of high saturated magneticflux density and high magnetic permeability can be obtained. Asdemonstrated by Example 11, if ferrite materials, which have highelectrical resistivity and high magnetic permeability, and inparticular, Mn-Zn ferrites or Ni-Zn ferrites, are employed as the secondsubstance, then composite materials with high magnetic permeability inthe high frequency range can be obtained.

EXAMPLE 15

A Ni-Fe alloy with a Ni:Fe at the weight ratio of 78.5:21.5 in the formof a spherically granular powder (mesh #250, mean particle size 30 μm)was prepared. Insulating films several nanometers in thickness, composedmainly of aluminum nitride, were formed on the surfaces of these powdergrains by sputtering for 5 minutes with an aluminum target in a nitrogenatmosphere.

This powder of coated grains was molded by compression at 500 kg/cm²,after which the green body was hot pressed in an atmosphere of argon gasat a temperature of 800° C. under a pressure of 1000 kg/cm², therebyforming a high-density (relative density 98-99%) composite magneticmaterial.

The magnetic characteristic (magnetic permeability versus frequency) ofthis composite magnetic material is shown in FIG. 6.

The electrical resistivity of the aforesaid composite magnetic materialwas high, i.e., 10⁷ -10⁸ ohm-cm, therefore, the magnetic permeability μ(CGS) was almost independent of frequency, exhibiting values of about1300-1400 throughout the range of 10 kHz to 1 MHz.

Next, the same raw material was compression molded after the addition ofboron oxide in a proportion of 0.05-0.10 wt % as a sintering aid, andthen hot pressed under the same conditions as described above. Highdensity was attained (relative density 99.5%), the thickness of theinsulating layer was approximately 7 nm, and high electrical resistivitywas also obtained. This composite material exhibited a magneticpermeability μ (CGS) of 1700-1800 even in the high frequency range.

EXAMPLE 16

A Ni-Al-Fe alloy with a Ni:Al:Fe in a weight ratio of 10:6:84 in theform of a spherically granular powder (mesh #250, mean particle size 30μm) was prepared. Insulating films 30 nm in thickness, composed mainlyof aluminum nitride, were formed on the surfaces of these powder grainsby heat treatment at 800° C. for one hour in a nitrogen atmosphere (flowrate 200 cc/min).

This powder of coated grains was molded by compression at 500 kg/cm²,after which the green body was hot pressed in an atmosphere of argon gasat a temperature of 900° C. under a pressure of 1000 kg/cm², therebyforming a high-density (relative density 98-99%) composite magneticmaterial.

This composite magnetic material exhibited high electrical resistivityρ, i.e., 10⁸ -10⁹ ohm-cm, while at the frequency of 1 MHz the magneticpermeability μ (CGS) and saturated magnetic flux density of thiscomposite were 1470 and 9500 gauss, respectively.

This example shows that, by using metal grains with aluminum as theprincipal component and heat-treating these grains in an atmosphere ofnitrogen gas, which is a simple method highly suitable for massproduction, uniform insulating films composed mainly of aluminum nitridecan be formed on the grain surfaces. Tests also confirmed that thesethin coating films have sufficiently high mechanical strength towithstand breakage at high temperatures and pressures.

EXAMPLE 17

Using the same spherically granular Ni-Al-Fe alloy powder as wasemployed in Example 16, a 5-10 nm thick insulating film composed mainlyof aluminum nitride was formed on the surfaces of these spherical powdergrains by the same method as that used in Example 16.

Boron oxide, lead oxide, bismuth trioxide, vanadium pentoxide, ormagnesium oxide were combined with this alloy powder as additives in theproportion of 0.01-0.50 wt %. The quantity of this additive was chosenso that, for example, a boron oxide film of thickness 2-100 nm inthickness would be formed on the surfaces of the aforesaid grains. Thismixture was molded by compression at 500 kg/cm², and then hot pressedfor two hours at a temperature of 900° C. under a pressure of 1000kg/cm², thereby obtaining a high-density (relative density 99%)composite magnetic material.

The thickness of the thin coating film constituting the continuous phaseof this composite magnetic material was 5-50 nm, which was the combinedthickness of the aluminum nitride and the boron oxide, lead oxide,bismuth trioxide, vanadium pentoxide, or magnesium oxide. All of thesecomposite materials exhibited high electrical resistivities ρ, rangingfrom 10⁸ to 10⁹ ohm-cm. FIG. 6 shows the relationship between thefrequency and the magnetic permeability with respect to the compositematerial obtained when boron oxide was used as the additive. As seenfrom FIG. 6, the magnetic permeability μ (CGS) of this compositematerial at frequency of 1 MHz was 2000. The saturated magnetic fluxdensity of the composite material was 9600 gauss. Similar results wereobtained when lead oxide, bismuth trioxide, or vanadium oxide were usedin place of boron oxide, confirming that all of these substances areeffective as sintering aids. However, when magnesium oxide was used forthis purpose, the magnetic permeability μ (CGS) ranged from about 1000to 1500. Also, when the film thickness exceeded 50 nm, the value of μdecreased below 1000.

Comparison of the present results with those of Example 16 shows thatthe addition of lead oxide, bismuth trioxide, vanadium pentoxide, orboron oxide provides high electrical resistivity even for thinner films,therefore, increases in the value of the magnetic permeability μ due tomagnetic resistance can be suppressed, and hence high permeabilities canbe obtained even in the high frequency range.

Next, aluminum oxide insulating films of thickness 5-30 nm were formedon the grain surfaces of Si-Al-Fe alloy powder, and to these coatedgrains was added a quantity of boron oxide sufficient to form additionalcovering films with a thickness of 10-20 nm on the surfaces of thecoated grains. This mixture was hot pressed under the conditions statedabove, thereby obtaining a high-density composite material. Thiscomposite material exhibited high electrical resistivity ρ, i.e., 10⁸-10⁹ ohm-cm, as well as high magnetic permeability μ (CGS), i.e., about2500. However, when the other additives (bismuth trioxide, vanadiumpentoxide, or lead oxide) were used, the value of the magneticpermeability μ (CGS) was about 2000, showing that boron oxide isparticularly suitable for the present purpose. In addition, for othermagnetic alloys, in particular, iron alloys containing aluminum, highmagnetic permeabilities were obtained by using combinations of aluminumoxide and boron oxide, aluminum nitride and boron oxide, or aluminumoxide nitride and boron oxide in the above process.

EXAMPLE 18

This example will describe a structural material.

First, an aluminum powder consisting of grains with a mean particle sizeof 20 μm was subjected to superficial oxidation treatment by heating at600° C. for ten hours in an air stream at a flow rate of 200 cc/min,thereby forming an insulating film several tens of nanometers inthickness, composed mainly of Al₂ O₃, on the surfaces of the powdergrains. These coated powder grains were then molded into a disk-shapedsample by compression at 500 kg/cm².

The green body sample so obtained was maintained at a temperature of600° C. in air for a period of 30 minutes, and then hot pressed at 500kg/cm², this pressure being maintained for 30 minutes, after which thepressure was released and the sample was cooled, thereby obtaining adisk-shaped composite material of diameter 13 mm and thickness 9.6 mm.

The electrical resistivity of the composite material of this example was6×10⁹ ohm-cm. As for mechanical properties, the ductility of thiscomposite material was found to be virtually the same as that ofconventional aluminum materials. The thermal conductivity of thiscomposite material was about 200 W/m-deg, which is higher than that ofconventional Al-Al₂ O₃ composite materials.

EXAMPLE 19

This example will describe a composite material for use in magneticcores.

Substantially spherical grains composed of Sendust (an Fe-Si-Al alloy)with a mean particle size of 20 μm, prepared by gas atomization, wereheat-treated at temperatures from 850° C. to 950° C. for periods rangingfrom one to ten hours in atmospheres with various concentrations ofoxygen. The weight changes which occurred in this process were measured.Also, Auger electron spectroscopic analysis of the surfaces of thesepowder grains revealed that, in each case, aluminum oxide was the maincompound formed during this treatment.

Next, this powder was sealed in an airtight vessel together with oxygen,after which the material was sintered with a hot isostatic pressapparatus for three hours at a temperature of 800° C. under a pressureof 2000 kg/cm². The surfaces of the sintered bodies so obtained weremirror polished, and the surface electrical resistance was measured witha tester over an arbitrarily selected interval of approximately 10 mm.Next, samples 0.5 mm in thickness were prepared, and the magneticpermeability of these samples was measured at the frequency of 1 MHz.The results of these measurements are shown in Table 2. The same testswere also applied to sintered bodies prepared from the same alloy powderwhich had not undergone the initial heat treatment described above; theresults for these samples are also shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Weight Increase                                                                             Electrical                                                                              Magnetic                                              of Grains     Resistance                                                                              Permeability (CGS)                                    (%)           (ohm)     (at 1 MHz)                                            ______________________________________                                        Not heat treated                                                                            ˜0   70                                                    0.010        5         700                                                   0.10          20        1500                                                  0.5           50        1000                                                  1.0           700 k     800                                                   1.8           >20M      700                                                   2.5           >20M      100                                                   3.0           >20M       80                                                   ______________________________________                                    

The data in Table 2 shows that greater weight increase of the powder isassociated with greater electrical resistance. If the weight increase is1.8% or greater, then the sintered body so obtained, althoughessentially a bulk metal in composition, exhibits electrical resistanceclose to that of an insulator.

Measurements of the magnetic permeability of samples with a thickness of0.5 mm at the frequency of 1 MHz revealed that the magnetic permeability(CGS) of samples prepared from untreated powder was low, i.e., about 70,due to eddy-current losses. The powder with a weight increase of0.10-0.5% due to heat treatment exhibited nearly the maximum magneticpermeability, and weight increases beyond this range actually resultedin diminished magnetic permeability, which in fact, decreased to the lowvalue of 80 for samples with 3% weight increase. This is attributed tothe greater resistance to penetration of magnetic flux by thicker layersof nonmagnetic material (oxide layers) as well as loss of magneticproperties by the magnetic phase itself as a result of oxidation.

As shown by the schematic illustration in FIG. 1, the morphologicalstructure of the sintered bodies (composite materials) obtained in thisexample comprises a discrete phase including grains 1 made of the firstsubstance (a magnetic metal material) and a continuous phase including athin coating film 2 made of the second substance (an insulating film, inthe present case, aluminum oxide). In actual practice, the thickness ofthis insulating layer 2 is several tens of nanometers. The size of thegrains used in such a composite material should be appropriatelyselected according to the frequency band in which the magnetic core isto be used, i.e., according to the degree of eddy-current loss. Thepowder is loaded in an airtight vessel containing oxygen when hotisostatic pressing is performed so that even if the insulating layer isbroken by plastic deformation of the powder grains during the process ofsintering, oxygen is replenished and an oxide film (insulating film) isreformed at the damaged portions. As a result, passage of currentbetween the metal grains is suppressed. Although oxygen gas was used forthis purpose in this example, any active gas which reacts with the firstsubstance and thereby produces the second substance or a thirdinsulating substance can also be used in place of oxygen.

The process of sintering by means of heat and isostatic pressure whichwas used in the preparation of this example is advantageous in that thegrains undergo isotropic deformation and therefore the insulating filmis relatively insusceptible to breakage. The weight increase of thegrains resulting from reaction with the active gas should desirably bein the range from 0.01% to 2.5%. If this weight increase is 0.01% orless, then the insulation effect is often inadequate, whereas if theweight increase amounts to 2.5% or more, then the magneticcharacteristics deteriorate.

The porosity of the composite material so obtained should desirably notbe more than 3%, since composite materials with a porosity of 3% or lesshave sufficient mechanical strength. Even in cases where the compositematerial is to be used in magnetic heads, adequate performance is notsecured if the porosity amounts to 3% or more.

Although an Fe-Si-Al alloy was used in this example, similar results areobtainable if other alloys are used, for example, any appropriate alloycontaining aluminum and silicon.

EXAMPLE 20

An Si-Al-Fe alloy composed of 10 wt % silicon, 6 wt % aluminum, and 84wt % iron in the form of a spherically granular powder (mean particlesize approximately 30 μm) was heat-treated in air at a temperature of600° C. for 5 minutes, thereby forming a coating film on the surfaces ofthe powder grains. The thickness of this coating film was estimated fromthe weight increase which occurred during heat treatment and the resultsof Auger electron spectroscopy as well as the argon sputtering depthprofile, and was found to be approximately 9 nm.

The powder was molded and hot pressed in an atmosphere of argon at atemperature of 800° C. and a pressure of 1000 kg/cm², thereby preparingthe primary samples. Several of these primary samples were prepared byvarying the hot pressing time between 5 minutes and 2 hours. Densitymeasurements of these primary samples showed that the relative densitieswere 78% for those hot pressed for five minutes, 90% for those hotpressed for twenty minutes, and 98% for those hot pressed for two hours,with respect to the theoretical density of 6.89 g/cm³. These threevarieties of samples were designated as Nos. 1, 2, and 3, respectively.Next, the following three types of treatment were applied to each ofthese three varieties of molded samples, and the resulting samples wererespectively designated as Nos. 4-6, Nos. 7-9 and Nos. 10-12.

Samples Nos. 4-6 were prepared from samples Nos. 1-3, respectively, byheat-treating in air at 700° C. for approximately 10 minutes, followedby vacuum packing into airtight vessels.

Samples Nos. 7-9 were prepared from samples Nos. 1-3, respectively, byloading into airtight vessels together with approximately 10 cc ofoxygen.

Samples Nos. 10-12 were prepared as follows. Three 100 ml aliquots of a0.01M ethanol solution of titanium ethoxide were prepared, and theaforesaid samples Nos. 1-3 were added to the three respective aliquots.Heat refluxing was performed at 70° C. while a 1:3 water-ethanol mixturewas added dropwise to each solution until the concentration of waterreached 0.002M after three hours. After completion of this process, theproducts were filtered, then the powders so obtained were dried at 150°C. and heat-treated in air at 400° C. for one hour. The powder samplesobtained in this manner were then vacuum packed into airtight vessels.

The first type of treatment described above was performed for thepurpose of increasing the thickness of the insulating film around thepores of the primary samples. The second type of treatment describedabove was performed for the purpose of filling the pores of the primarysamples with oxygen gas in order to induce a reaction between this gasand the metal at the pore sites during the high-pressure molding step(described below) and thereby increase the thickness of the insulatingfilm. The third type of treatment described above was performed for thepurpose of forming a titanium dioxide film on the surfaces of the poresof the primary samples.

The nine varieties of samples obtained in this manner were subjected toisostatic pressing for one hour at 800° C. and 2000 kg/cm² in an argonatmosphere, thereby obtaining the final secondary samples. Then, 2×1×12mm prismatic test samples were cut from the high-density compositematerial so obtained, following which the specific gravity, electricalresistance (using a tester), and magnetic characteristics of these testsamples were measured, with the results shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Physical properties of composite materials                                    Density        Elec-   Saturated                                              (g/cm.sup.3)   trical  Magnetic  Magnetic                                            Pri-    Sec-    Resis-                                                                              Flux    Permeability                             Sample mary    ondary  tance Density (CGS)                                    No.    Sample  Sample  (ohm) (kilogauss)                                                                           1 kHz 1 MHz                              ______________________________________                                        1      5.37    --      >20M   8       280  240                                2      6.34    --      >20M  10       670  570                                3      6.75    --      1.0   11      1120  430                                4      5.37    6.86    0.2   11       970  350                                5      6.34    6.85    >20M  11      1020  970                                6      6.75    6.85    <0.1  11      1100  120                                7      5.37    6.84    0.4   11       850  230                                8      6.34    6.84    >20M  11      1060  990                                9      6.75    6.83    <0.1  11      1080  110                                10     5.37    6.77    2 k   11       690  470                                11     6.34    6.79    >20M  11      1000  930                                12     6.75    6.82    <0.1  11      1140   70                                ______________________________________                                    

As can be seen from Table 3, among samples Nos. 1-3, which were notsubjected to a second pressurizing treatment, the higher sample No. 3(density 98%) exhibited the lowest electrical resistance. Moreover,sample No. 3 exhibited a high magnetic permeability at 1 kHz, but themagnetic permeability at 1 MHz was less than half of the value at 1 kHz.The lower density samples Nos. 1 and 2 had high electrical resistance,and consequently the frequency dependence of the magnetic permeabilityin these samples was slight. However, due to their low density, thesesamples exhibited low saturated magnetic flux density as well as lowmagnetic permeability.

Samples obtained by applying further high-pressure sintering after theabove-described treatment had been applied to these primary samples, allexhibited low electrically insulating resistance, except for that withprimary sinter density 90%, and consequently the frequency dependence ofthe magnetic permeability became very pronounced. On the other hand, theelectrical resistance of the sample with primary density 90% was notgreatly lowered by further densification, and therefore the resultingcomposite material exhibited high magnetic permeability even up to thehigh frequency range.

For purposes of comparison, the aforesaid three varieties of primarysamples were also vacuum packed into airtight vessels without anyadditional treatment, and composite materials were then obtained fromthese samples by applying isostatic pressure for one hour in an argonatmosphere under the same conditions. Evaluation of the properties ofthese composite materials revealed that all materials had low electricalresistance and that their magnetic permeability exhibited a pronouncedfrequency dependence.

In general, even if the surface of a metal powder is completely coveredby an electrical insulator, if this material is then compressed so as toform a molding of density nearly 100%, then the deformation of the metalpowder grains causes a change in surface area, and a portion devoid ofan insulating film is formed. As a result, the metal grains come intomutual contact, and this presumably causes a decrease in electricalresistance. As was demonstrated by the results concerning samples Nos.1-3, destruction of the insulating film does not occur unless thedensity becomes quite high. However, by the method of this example, highdensity as well as high electrical insulation characteristics can beattained only with he formation of insulating substance at the siteswhere destruction of the insulating film is prone to occur and withoutincreasing the overall quantity of the insulating substance.

In addition to those described in this example, the inventors have alsoprepared samples with various primary sintered densities, and haveascertained that relative densities in the range from 80% to 95% aremost advantageous for the present purpose. If the density of the primarysintered bodies in excessively high, then the pores are closed, whichimpedes filling with an insulating substance or a compound which canform an insulating substance by an appropriate chemical reaction. On theother hand, if the density of the primary sintered bodies is unduly low,and if a large quantity of an electrically insulating substance isadded, then the fraction of the resulting composite material occupied bythe insulating substance is unduly large, which results in low magneticpermeability, while conversely, if the quantity of electricallyinsulating substance added is small, then the insulation resistance islikely to be low.

EXAMPLE 21

First, an aluminum metal powder consisting of grains with a meanparticle size of 20 μm was heat-treated in air at a temperature of 400°C. for two hours, thereby forming an oxide film on the surfaces of thepowder grains. The thickness of this coating film was estimated from theweight increase which occurred during heat treatment and the results ofAuger electron spectroscopy as well as the argon sputtering depthprofile, and found to be approximately 10 nm. This powder was molded,sealed into a metal aluminum pipe in a vacuum state, and then subjectedto isostatic compression in an atmosphere of argon gas for one hour at atemperature of 600° C. under a pressure of 500 kg/cm², thereby obtainingprimary sintered samples with a relative density of 92%.

These primary sintered samples were then heat-treated in air at 400° C.for two hours. The resulting samples were sealed into metal aluminumpipe in a vacuum state and then subjected to isostatic compression in anatmosphere of argon gas for three hours at a temperature of 600° C.under a pressure of 1000 kg/cm², thereby obtaining the secondarysintered samples. For purposes of comparison, secondary sintered sampleswere also prepared under the same conditions from the primary sinteredsamples obtained as described above, which had not been subjected to theaforesaid additional heat treatment at 400° C.

Next, 2×1×12 mm prismatic test samples were cut from the secondarysintered samples so obtained, following which the density, electricalresistance (using a tester), and thermal conductivity of these testsamples were measured. The results of these measurements showed that thedensity of all these samples was 97%, but the electrical resistance ofthe samples which had not undergone the additional heat treatment at400° C. was low, i.e., of the order of 50 ohm, whereas the electricalresistance of the sample which had received the additional heattreatment at 400° C. was 20 megohm or more. The thermal conductivity ofthese samples was approximately 180 W/m-deg, which is close to that ofaluminum.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. A composite sintered material with a resistivityof at least 10 ohm-cm comprising:a discrete phase including grains madeof a first substance, said first substance comprising at least one of ametal and an alloy thereof; and a continuous phase including a thincoating film made of a second substance of a dielectric materialcomprising an inorganic material and formed on the surface of saidgrains to prepare coated grains, said thin coating film having a meanthickness smaller than the mean particle size of said grains, whereinsaid grains are separated substantially from each other by said thincoating film and the porosity of said composite sintered material is 5%or less, said coated grains being compacted into a green body and saidgreen body being densified in an active gas atmosphere of oxygen,nitrogen or air, and said mean thickness of the thin coating film beingfrom 5 to 50 nanometers, and wherein said first substance contains analuminum metal and said second substance of a dielectric materialcontains oxides or nitrides of aluminum.
 2. A composite sinteredmaterial according to claim 1, wherein said first substance is at leastone of a magnetic metal and a magnetic metal alloy thereof.
 3. Acomposite sintered material according to claim 1, wherein said secondsubstance of a dielectric material contains at least one member selectedfrom the group consisting of boron, lead, vanadium, and bismuthcompounds.
 4. A composite sintered material according to claim 1,wherein said first substance is at least one of a metal and an alloythereof and said second substance of a dielectric material is a materialwith a substantial electrically insulating nature comprisingsuperplastic ceramics.
 5. A composite sintered material according toclaim 4, wherein said super-plastic ceramics contain at least one memberof the group consisting of apatite and zirconium oxide.
 6. A compositesintered material according to claim 1, wherein said grains have aplatelet shape.
 7. A composite sintered material with a resistivity ofat least 10 ohm-cm comprising:a discrete phase including grains made ofa first substance, said first substance comprising at least one of ametal and an alloy thereof; and a continuous phase including a thincoating film made of a second substance of a dielectric materialcomprising an inorganic material and formed on the surface of saidgrains to prepare coated grains, said thin coating film having a meanthickness smaller than the mean particle size of said grains, whereinsaid grains are separated substantially from each other by said thincoating film and the porosity of said composite sintered material is 5%or less, said coated grains being compacted into a green body and saidgreen body being densified in an active gas atmosphere of oxygen,nitrogen or air, and said mean thickness of the thin coating film beingfrom 5 to 50 nanometers, and wherein said first substance is aniron-based magnetic metal and said second substance of a dielectricmaterial is selected from manganese-zinc ferrite or nickel-zinc ferrite.8. A composite sintered material with a resistivity of at least 10ohm-cm comprising:a discrete phase including grains made of a firstsubstance, said first substance comprising at least one of metal and analloy thereof; and a continuous phase including a thin coating film madeof a second substance of a dielectric material comprising an inorganicmaterial and formed on the surface of said grains to prepare coatedgrains, said thin coating film having a mean thickness smaller than themean particle size of said grains, wherein said grains are separatedsubstantially from each other by said thin coating film and the porosityof said composite sintered material is 5% or less, said coated grainsbeing compacted into a green body and said green body being densified inan active gas atmosphere of oxygen, nitrogen or air, and said meanthickness of the thin coating film being from 5 to 50 nanometers, andwherein said first substance is an iron-based magnetic metal containingaluminum and said second substance of a dielectric material is at leastone selected from the group consisting of aluminum oxide, aluminumnitride, and aluminum oxide nitride.